A modulated grating Y-branch (MG-Y) tunable semiconductor laser can achieve fast tuning in a wide wavelength range and is expected to become one of the most promising light sources in fiber optic sensing applications. In order to meet the demand for fine wavelength quasi-continuous tuning in fiber optic sensing applications, this paper proposes an automated test technique for MG-Y lasers based on spline interpolation. This scheme makes full use of the tuning characteristics of the MG-Y laser. A smooth tuning path covering the range of 40 nm is obtained by coarse scanning of the currents in the reflection areas of the left (IRR) and right (ILR) gratings. After the deduplication and stitching of the linear tuning sections corresponding to each IRR and ILR pair, fast interpolation retrieval can be realized. Through the double calibration of current of the semiconductor optical amplifier and current in the phase region, the flat laser powers at different output wavelengths are realized. Using this automated calibration technique, a wavelength-current look-up table covering 1527 nm to 1567 nm and with a wavelength interval of 8 pm is constructed. The look-up table has smooth current tuning paths and the power fluctuation is less than 0.2 dBm, which can be used in fiber optic sensing applications requiring fast spectrum acquisition.